Arlene Foster and Martin McGuinness have warned that the UK's exit from the EU must not undermine the war against cross-border crime.

Despite taking opposing positions over the UK's position in the European Union, the First and Deputy First Ministers joined forces to warn Prime Minister Theresa May how Northern Ireland could suffer badly if the Brexit wasn't handled carefully.

"There have been difficult issues relating to the border throughout our history and the peace process", they wrote in a letter.

"We therefore appreciate your stated determination that the border will not become an impediment to the movement of people, goods and services.

"It must not become a catalyst for illegal activity, or compromise in any way the arrangements relating to criminal justice and tackling organised crime.

"It is equally important that the border does not create an incentive for those who would wish to undermine the peace process and/or the political settlement".

Dissident republican groups use the Republic as a preparatory base for attacks north of the border, while cross-border smuggling can be a lucrative source of funding.

Stormont's top two politicians also warned the Conservative leader that the province's agri-food industry is particularly vulnerable to the loss of EU funding.

EU funds have been "hugely important" to the peace process and the province's economy, amounting to £13bn from 1994, Mrs Foster and Mr McGuinness said. A proportion of EU funds for projects in Northern Ireland may not be drawn down due to the exit, which Mrs Foster and Mr McGuinness said was of real concern.

The leaders said they wished to play a role in the engagement between the British and Irish governments on matters concerning the border.

However, Ulster Unionist economy spokesperson Steve Aiken MLA said the areas of concern which were outlined all existed before the referendum.

"I'm astonished the ministers would have the audacity to release this letter," he said.

"Indeed, given the content of the letter I would question whether this is a DUP U-turn on their position on the referendum after the vote has taken place?

"It is about time the Executive came forward with real plans for dealing with Brexit. Instead of providing clarity and solutions, the Executive Office is quickly becoming the Department for Stating the Obvious."

Meanwhile SDLP MLA for south Belfast Claire Hanna says the letter is "breath-taking reverse ferret" from the First Minister and signals confusion within Sinn Fein.

She said: "The First Minister has serious questions to answer over the breath-taking reverse ferret she is currently performing. Ms Foster has clearly realised the error of her ways in backing Brexit and now the horse has bolted, is asking the Conservative Government to close the gate.

She continued: "Once again Sinn Fein is taking its lead from the DUP in the Executive. It failed to fully mobilise behind the campaign to remain in the EU and appears unable to stand up to the DUP by fighting for the democratic will of the people by going along with Arlene's determination to see Article 50 triggered and the North dragged out of Europe.

"It must have slipped Martin McGuinness' mind that a senior Sinn Fein MLA has signed up to the same legal challenge that the SDLP are committed to, along with a number of other parties and agencies, to prevent the North from being removed against our will. It is also a substantial oversight that this letter was not copied to Enda Kenny as there is a substantial North South Dimension.

"What is glaringly obvious is the Executive has no plan for how to deal with any of these issues. This matter is too critical to be left to those who failed to understand the issue in the first place or someone who can't make up his mind. The SDLP is calling for the urgent development of a Brexit Committee in the Assembly to fully explore the options available to us."